1. Field of the Invention
The present invention relates to monitoring of undesirable fluid ingress into subsea control modules.
2. Description of the Related Art
The hydraulic and electronic components of a subsea well, such as a hydrocarbon extraction well, are typically housed in a sealed vessel termed a subsea control module (SCM), located on a Christmas tree which is mounted on the sea bed above the well bore. An SCM is, typically filled with electrically insulating oil, to alleviate the need to design it to withstand high pressures and provide a first line of defense for the control system against the seawater environment. Typically, an SCM houses hydraulic manifolds, directional control valves and a subsea electronics module (SEM) which is itself a sealed unit. Thus, ingress of seawater resulting from a leak in the SCM housing, or ingress of hydraulic fluid from a small leak in the hydraulic system, will not in itself cause a malfunction of the control system. However, well operators, historically, have needed to know if there is a seawater leak since this will result in corrosion of the components in the SCM and possible failure of the system earlier than expected.
Existing arrangements consist of sets of metal electrode pairs mounted on an insulating panel or a metal frame with insulating inserts within the SCM, typically four, at equal intervals between the bottom and the top, each connected to an operational amplifier via a low voltage source and a series resistor, thus enabling the detection of the presence of the ingress of electrically conductive seawater, and, in a crude manner, the degree of displacement of the original oil filling. A typical application of the technique is illustrated diagrammatically in FIG. 1 in which a casing 1 of an SCM is shown as a transparent outline to show an electrically insulating panel 2, mounted on the base 3 of the SCM, with pairs 4 of electrodes mounted on it. FIG. 2 shows circuitry 5 around an operational amplifier 6, typically housed in the SEM within the SCM, to which the electrode pairs, are connected. Since the resistance across a pair of electrodes when in contact with seawater is very low, i.e. effectively a short circuit, they are shown in a block 7 as simple switch contacts 8, 9, 10 and 11. One of the electrodes of each pair is connected to a voltage source V, typically 2 volts. The gain of the circuit 5 is the ratio of the resistance of a feedback resistor 12 across amplifier 6 and the effective resistance provided by input resistors 13, 14, 15 and 16, each of which is in series with a respective one of the electrode pairs 4 and an input of amplifier 6. Each of the input resistors is chosen to be of a resistance which is one quarter of that of the feedback resistor 12. Thus, if there is water ingress into the casing 1 to the level of the lowest electrode pair, then the contacts 8 of the block 7 are effectively closed and the output 17 of the operational amplifier 6 will rise to ¼ of V. Likewise, further ingress of seawater reaching the remaining electrode pairs will result in the output 17 rising to ½ V, ¾ V and V respectively as the contacts 9, 10 and 11 become effectively closed. Thus, a crude indication of the seawater ingress level is obtained by the electronic circuitry of the SEM reading the output 17 of the circuit 5 and transmitting the information topside, as a digitised version of the analogue signal, to the well operator, typically via the well umbilical, as part of the well housekeeping/diagnostic telemetry.
A problem with the existing technique is that it is unable to detect the ingress of hydraulic fluid into the SCM resulting from a leak in the hydraulic system in the SCM. Currently, a well operator has relied on hydraulic fluid leak detectors at the fluid source but these cannot confirm whether the leak is actually within the SCM. A further problem is that current flow through the electrode pairs results in their corrosion.
This invention enables the detection of both the ingress of seawater and hydraulic fluid in the SCM and provides a better indication of the degree of ingress, and reduces corrosion of the sensing electrodes.
Recent measurements have been made in the laboratory of the change in conductivity of the insulating oil in an SCM with contamination by the hydraulic fluid used for the well control system, which is a glycol based trans-aqua fluid. Results show that the conductivity of the contaminate in the oil is much less than that due to seawater and thus the existing contamination detection technique described before was not sensitive enough to be able to detect the ingress of trans-aqua fluids. Measurements have also shown that sea-water and trans-aqua hydraulic fluid (glycol) in insulating oil result in an immiscible fluid with both contaminants having a greater density than the oil. Thus, ingress of these contaminants displaces the transformer oil from the base of the SCM upwards. This invention provides an improved method for the monitoring of undesirable fluid ingress into an SCM to enable detection of the ingress of trans-aqua hydraulic fluid as well as seawater, whilst still providing a zoned measure of the degree of ingress and using a variety of methods of measurement which also reduces, substantially, corrosion of sensing electrode pairs.